1. Field of the Invention
The present invention relates generally to a system and method for enhancing particulate collection from the gaseous exhaust stream of an industrial process, and more particularly to such a system and method whereby the collection is enhanced by charging the particulate and utilizing electrical forces to increase collection in a packed bed scrubber system.
2. Description of the Prior Art
Many industrial processes, particularly thermal processes such as the incineration of waste materials or high temperature production of materials such as glass fibers, emit small or sub-micron-sized particulate in their gaseous exhaust stream, which particulate is normally considered hazardous by the Environmental Protection Agency and regulated as such. Accordingly, there has long been a need for systems and methods for removing such particulate from the exhaust stream prior to its entry into the atmosphere.
Various systems for electrostatically charging the particulate have been developed, such as described in U.S. Patent App. No. 20040139853 which was published on Jul. 22, 2004 in the name of Bologa; et al. for “Apparatus for the electrostatic cleaning of gases and method for the operation thereof” discloses an apparatus comprising three-conduit sections: a ionization and cleaning section in which the particles contained in water-saturated air are ionized and then conducted through a chamber with grounded walls so that part of the particles are deposited on these walls; an additional cleaning section which includes grounded tubes past which the gas is conducted to remove additional charged particles; and a filter section in which dry remaining fine particles are removed from the gas stream.
It should be appreciated that such systems for electrostatically charging particulate matter has long been known in the industry. For example, U.S. Pat. No. 5,395,430 which issued to Lundgren, et al. on Mar. 7, 1995 for “Electrostatic precipitator assembly” discloses an electrostatic precipitator assembly including a tubular collector and an electrode suspended therein, wherein the electrode includes a substantially cylindrical collector portion and a charging portion which includes a rod and a charging disk, and further wherein the gap between the charging disk and the collector is at least as great as the gap between the collector portion of the electrode and the collector.
Two more examples of electrostatic cleaning systems are shown in U.S. Pat. No. 5,364,457 which issued to Cameron on Nov. 15, 1994 for “Electrostatic gas cleaning apparatus” and U.S. Pat. No. 5,282,885 which issued to Cameron on Feb. 1, 1994 for “Electrostatic gas cleaning process and apparatus” both of which disclose processes and apparatuses for collecting particles or droplets in which a charging device and condensation equipment are combined to provide a cleaning apparatus that operates at a cost less than conventional apparatuses.
Another example is U.S. Pat. No. 4,265,641 which issued to Natarajan on May 5, 1981 for “Method and apparatus for particle charging and particle collecting” which discloses a method and apparatus for charging and collecting submicron particles whereby the particles are charged by a needle-to-plate ionizer having offset rows of needles which are spaced from the plate. Charged particles are collected in a collecting section having a deflector electrode and a pair of collecting plates wherein the deflector electrode includes a conductor embedded in a dielectric material having a dielectric constant greater than 1, which dielectric material suppresses arcs between the deflector electrode and the collecting plates.
In yet another example, U.S. Pat. No. 4,222,748 which issued to Argo, et al. on Sep. 16, 1980 for “Electrostatically augmented fiber bed and method of using” discloses an apparatus including a grounded fiber bed of 50 to 1000 micron average diameter fibers packed to a bed, an electrostatic or ionizing field means upstream of the fiber bed to place an electrical charge on the particulates, and irrigation means for the fiber bed, and optionally the grounded electrodes of the electrostatic means as well, to flush collected particulates from the fiber bed and optionally from the grounded electrodes. In operation, particulates are charged in the electrostatic means and the charged particulates are collected in the fiber bed where the electrical charge is dissipated through the irrigating liquid/particulates mixture so that no significant space charge effect is allowed to develop in the fibers of the fiber bed and re-entrainment of particulates is avoided.
The use of a venturi to increase the velocity of the gas stream was taught in U.S. Pat. No. 4,110,086 which issued to Schwab, et al. on Aug. 29, 1978 for “Method for ionizing gases, electrostatically charging particles, and electrostatically charging particles or ionizing gases for removing contaminants from gas streams,” which discloses the use of a venturi to increase the velocity of contaminated gases and guides the gases past a high, extremely dense electrostatic field presented perpendicular to the gas flow and extending radially outward between a central, accurately sized disc electrode and the surface of the venturi throat. Downstream, charged particles are collected by a wet scrubbing process or electrostatic precipitator. A similar device is disclosed in U.S. Pat. No. 4,093,430 which also issued to Schwab, et al. on Jun. 6, 1978 for “Apparatus for ionizing gases, electrostatically charging particles, and electrostatically charging particles or ionizing gases for removing contaminants from gas streams.”
Similarly, U.S. Pat. No. 4,072,477 which issued to Hanson, et al. on Feb. 7, 1978 for “Electrostatic precipitation process” discloses an electrostatic precipitator which operates on the principle of mutual repulsion of charged particles to a grounded wall wherein the solid particle laden gas stream enters a collecting section where additional particles in the form of droplets, normally water, are injected in the form of a fine spray into the solid particle laden gas stream, and the solid particles and the additional particles are electrostatically charged either by conventional corona or by injecting the droplets from a charged nozzle and as the charged particles pass through the grounded section of the precipitator, a fraction of the water particles and solids are forced to the grounded wall by electric fields created by the space charge. Precipitated solid particles are entrained in the coalesced water which runs down the walls and is drained from the precipitator.
In the 1970s, Ceilcote APC developed the Ionizing Wet Scrubber (IWS) to address sub-micron particulate removal from gaseous emission streams. The IWS system is described, in U.S. Pat. No. 3,958,958 which issued to Klugman, et al. on May 25, 1976 for “Method for electrostatic removal of particulate from a gas stream” discloses a method including a packed wet scrubber through which a scrubbing liquid such as water is flowed vertically downwardly and through which gas to be cleaned is flowed in a direction transverse to the direction of flow of the scrubbing liquid. The stream of gas to be treated is ionized prior to its flow through the wet scrubber to provide particles in the gas stream with an electrical charge of a given polarity, and upon flow of the gas stream through the wet scrubber, the charged particles in the gas stream are carried into close proximity with and are attracted to the scrubbing liquid and/or packing elements as a result of attraction forces between the charged particles and the electrically neutral packing elements and liquid. A similar device was disclosed in U.S. Pat. No. 3,874,858, which issued to Klugman, et al. on Apr. 1, 1975.
The IWS system combined an electrostatic charge section followed by a packed bed collection system. This system was very complex and expensive to operate. Other electrostatic collection methods have been utilized, but they fall short when collecting particulate in the sub-micron size range. Tri-Mer has developed a Cloud Chamber Scrubber (U.S. Pat. Nos. 5,147,423, 5,941,465) which utilizes ionization of particulate in a mesh electrode, followed by collection on finely atomized liquid droplets.
As shall be appreciated, the prior art fails to specifically address either the problem or the solution arrived upon by applicant.